Exchange DAG

Dan130
Dan130 used Ask the Experts™
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I would like to configure a secondary exchange server and create a DAG between them in the UK office for DR purposes. Is this a task that requires downtime or is this quite straight forward. can i also create the DAG with them being on the same subnet? basically in the even that EX01  needs to re restarted i need EX02 to take over, any general advice regarding this would be helpful. we are running exchange 2010 on server 2008.
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Hi, for installation of second MB Server and DAG creation does not require downtime.
But its recommended to use 'Maintenance window' time for this activity.

During maintenance window, database seeding(Replication to other node will be faster).
Hi,

Using DAG, you can create copies of mailbox database in another mailbox server which is part of the same DAG. So if first server goes down, copy of the database in the second server is made active automatically.

DAG uses Microsoft Fail-Over clustering the background. Database will be replicated from first server to second server continuously. It is advised to configure a dedicated NIC on each server for Database Replication. Both servers should have a similar storage allocation as well.

You can create DAG member servers within the same subnet.

Following article has detailed information regarding DAG.

http://www.msexchange.org/articles-tutorials/exchange-server-2010/high-availability-recovery/uncovering-exchange-2010-database-availability-groups-dags-part1.html
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Author

Commented:
Great Thanks, I have been trying to distinguish if i also need to create a CAS array also.  we are currently 100 users and thought the DAG would give me the failover access in case the primary exchange goes down (I can still send and receive emails) is this the case and do i need a CAS array, in black and white why would i need a CAS array what does that give me in terms of DR.
DAG handles availability of Exchange database (Back-end).
CAS handles authentication of all mails clients.
and CAS array handles continuous mail access by balancing mail rquests through CAS.
i.e if One cas server gets overloaded , then new requests are diverted to less loaded CAS server. So end user wont have impact of slow MAPI/Mails connectivity.

In short CAS array gives better/Faster connection from mail clients to mailbox.

Author

Commented:
very help Manojbhoyar123 at which point is it advised to implement a CAS Array? we are currently 100 users and i don't see any issue with the load on our exchange now.

with the DAG this will allow us to have one ex server go down and for us to still send and receive emails without users being affected?

Thanks,
For 100 users, since you are getting good connectivity, you can skip CAS array.

And with DAG. Even if one of ur mailbox server goes down, users can still send and receive mails.
BUT.. for this all the databases configured, should be part of DAG.
Most Valuable Expert 2014
Commented:
"For 100 users, since you are getting good connectivity, you can skip CAS array. "

I don't agree with that statement.

All Exchange 2010 implementations should have an RPC CAS Array in place. You don't have to use WNLB, but if you don't deploy a CAS Array then you will have problems moving clients between different CAS role servers, such as the introduction of a new or replacement CAS role server.

I would have had a CAS array in place from day one, before any users were put in place. The bigger you get the harder it is to deploy one.

A CAS Array is just a DNS entry and a modification to Exchange - nothing else. It does not increase or decrease the load on the CAS so does no harm, but can save you a ton of work later.

http://semb.ee/casarray

Simon.

Author

Commented:
Thanks Simon, so if regards to your last statement "A CAS Array is just a DNS entry and a modification to Exchange - nothing else. It does not increase or decrease the load on the CAS so does no harm, but can save you a ton of work later. " if this is all the CAS array is why does it become harder as you grow to deploy one as you would simply run the modifications then.

Thanks,
Most Valuable Expert 2014

Commented:
It becomes harder to deploy because you have more machines to modify.
If you had it in place from day 1 of the Exchange 2010 server then all machines would be using it already and you just have to use DNS changes to move servers.

If the Outlook client has not had a profile repair done, then it isn't using the CAS Array. If you are deploying a load balancer that can mean some traffic is balanced and some is not.

Simon.

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